The use of combinations of organic compounds containing at least two intramolecular carboxylic acid anhydride groups per molecule, in particular copolymers containing succinic acid anhydride, with polyhydroxyl compounds, as binders for lacquers and coating compounds has already been disclosed in EP-A-48 128 but the idea of applying the principle of this publication to the reaction of amines with anhydrides has failed to be realized because the reaction of amines with anhydrides proceeds very rapidly even at room temperature, leading to cross-linked products with splitting up of the anhydrides. The extremely short pot life therefore obtained has hitherto prevented the joint use of polyanhydrides and polyamines in coating systems.
One possibility of a solution is indicated in DE-OS 2 853 477, where mixtures of blocked polyamines and polyanhydrides are described which have a sufficiently long working life but harden to cross-linked products on addition of water. Ketimines and aldimines obtained by the reaction of polyamines with ketones or aldehydes are described there as suitable blocked polyamines.
The polyanhydrides mentioned there include compounds containing at least two cyclic carboxylic acid anhydride groups in the molecule, in particular products obtained by the reaction of polyols with cyclic dianhydrides using a ratio of hydroxyl groups to anhydride groups of 1:2, and copolymers of an unsaturated cyclic anhydride with polyunsaturated compounds or .alpha.-olefines.
The olefinically unsaturated compounds to be used for the preparation of the copolymers are only treated in very general terms in the general description given in DE-OS 2 853 477 and in particular there is no mention of the quantitative proportions of the individual monomers required for the preparation of the copolymers. The specific disclosure given in the examples of practical embodiment is limited to copolymers of butadiene oil and maleic acid anhydride in proportions of 1:1 and to copolmers of wood oil with maleic acid anhydride. These copolymers have, however, disadvantages since their combination with bisketimines or bisaldimines would result in badly discoloured products. Moreover, coating compounds containing unsaturated oils such as butadiene oil or wood oil as binder component result in coatings which tend to brittle and are not weather resistant.
It is also clear from the concrete example given in DE-OS 2 853 477 that dimethyl formamide is used as solvent for working up the binders specifically described there and unacceptable solids contents of only about 20% are employed.
Another possibility of increasing the working life lies in the use of oxazolanes instead of amines.
Compositions of oxazolanes and polyanhydrides which can be hardened with water are known in principle from DE-OS 2 610 406, where oxazolanes are combined with polyanhydrides for water curable sealing compounds and adhesive compounds.
Polyanhydrides described there as suitable include reaction products of polyunsaturated fatty acids with maleic acid anhydride and polyanhydrides of C.sub.3 to C.sub.6 alkyl (meth)acrylate and maleic acid anhydride, in particular of butylacrylate and maleic acid anhydride.
The concrete examples of systems described in DE-OS 2 610 406 still require a great deal of improvement if they are to be suitable for the productions of high quality colourless lacquer films of great hardness and a good resistance to solvents and chemicals. This applies both to the systems based on copolymers of maleic acid anhydride and butyl acrylate described in the specific examples and to the systems based on reaction products of maleic acid anhydride with multi-unsaturated fatty acid esters, which results in end products which are subject to yellowing.
The use of epoxy functional compounds in combination with polyamines for lacquers and coating systems has been disclosed e.g. in EP-A-179 954 and 183 463.
The reactivity of such binder combinations is, however, not entirely satisfactory, and when they are cured at room temperature, cross-linking is insufficient or at least requires very long reaction times, even when selected catalyst systems are added.
Hardenable compositions of soluble, hydroxyl functional acrylate copolymers, compounds containing at least two cyclic anhydride groups per molecule and compounds containing at least two epoxide groups per molecule are described in DE-OS 3 534 910. Neither compounds which contain both epoxide groups and anhydride groups nor amino functional cross-linking agents are disclosed in the said publication.
Coating compositions of anhydride functional copolymers and at least difunctional epoxides with the addition of catalysts are described in EP-A-225 097. This publication also fails to describe any compounds which contain both epoxide and anhydride groups and it does not mention any amino functional cross-linking agents.
It was an object of the present invention to provide new binder combinations suitable for the preparation of high quality systems which could harden under the influence of atospheric moisture and would have a sufficiently long pot life. The coatings obtained should be clear, colourless, resistant to yellowing and solvent resistant.
This problem has been solved by providing the binder combination described below.